A base station in a conventional communication system allocates a plurality of spread codes designed to be spread at a frequency domain to each cell or sector, and uses different codes for respective user terminals or respective channels. That is, the base station selects a predetermined spread code from among available spread codes that are not allocated to a user terminal or a channel, and allocates the selected spread code to a new user terminal or a new channel.
There are two types of frequency domain spread codes. One is an orthogonal code, and the other is a quasi-orthogonal code.
When a length of the frequency domain spread code in the orthogonal codes is considerably short compared to a variation of a channel gain at the frequency domain, a receiver may completely separate more than two code signals and receive the same without interference between the codes due to orthogonality when more than two respective codes for one subcarrier combination are modulated and transmitted.
There is interference between the quasi-orthogonal codes.
In the orthogonal code, there are a number of spread codes, the number corresponding to a given frequency domain spreading factor. However, since more spread codes may exist in the quasi-orthogonal code compared to the orthogonal code, more user terminals or channels may be accepted.
However, the orthogonal code is widely used since it is easy to be designed and realized, and it has an advantage in using an orthogonal variable spreading factor (OVSF).
When the orthogonal code is used as the frequency domain spread code in the MC-CDMA communication system, the base station may use any of given orthogonal codes in an ideal channel situation and a timing and frequency synchronization situation, and it will cause no difference.
However, when a signal distortion occurs at a frequency spreading domain due to a channel error or a synchronization error for the user terminal, the orthogonality between the spread codes is broken, and interference between the codes may occur.
The above information disclosed in this Background section is only for enhancement of understanding of the background of the invention and therefore it may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art.